1. Field of the Invention
This invention relates to antenna design and, more particularly, to dual-ridged and quad-ridged broadband horn antennas.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
An antenna is a device which can radiate or receive electromagnetic (EM) energy. An ideal transmitting antenna receives power from a source (e.g., a power amplifier) and radiates the received power into space. That is, electromagnetic energy escapes from the antenna and, unless reflected or scattered, does not return. A practical antenna, however, generates both radiating and non-radiating EM field components. An example of a non-radiating EM field component would be the portion of the accepted power that is returned to the source, or otherwise dissipated in a resistive load.
The performance of an antenna can be characterized in a variety of ways. First, the radiation efficiency of an antenna (or “antenna efficiency”) can be defined as the ratio of the amount of power radiated by the antenna to the amount of power accepted by the antenna (from a power source). The portion of the power accepted by the antenna, but not radiated, may be dissipated in the form of heat. Other antenna performance characteristics include the operating frequency bandwidth, gain, directivity and the antenna pattern.
As used herein, the “antenna radiation pattern” may be generally defined as the spatial distribution of a quantity, which characterizes the electromagnetic field generated by the antenna. The antenna pattern is usually given as a representation of the angular distribution (in spherical coordinates, θ and φ, at a fixed point, R, from the antenna) of one of the following quantities: power flux density, radiation intensity, directivity, gain, phase, polarization and field strength (electric or magnetic). For example, the “radiation pattern” of an antenna may represent the angular distribution of the radiated power flux density in the far field (i.e., the region of the field of an antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region). For sinusoidal steady state fields, the radiation pattern may be formed by plotting the real part of the radial component of the Poynting vector:
                              W          ⁡                      (                          θ              ,              ϕ                        )                          =                              1            2                    ⁢                      Re            ⁡                          (                                                [                                                                                    E                        ->                                            ⁡                                              (                                                  θ                          ,                          ϕ                                                )                                                              ×                                          H                      ->                                        *                                          (                                              θ                        ,                        ϕ                                            )                                                        ]                                ·                                                      a                    ^                                    R                                            )                                                          [                  EQ          .                                          ⁢          1                ]            where, {right arrow over (E)} and {right arrow over (H)} are vector phasor representations of the electric and magnetic fields, respectively. In other words, the radiation pattern can be described as the tendency of an antenna to radiate electromagnetic energy (including electric and magnetic field components) as a function of direction in the far field region.
Though the radiation pattern of an antenna may be presented as a 3-D plot over all spherical angles, it is often beneficial to provide 2-D “cuts” of the radiation pattern when examining quantitative information. These “cuts” are generally made along the so-called E- and H-planes of the EM field in the far field region. For a linearly polarized antenna, the E-plane is the plane containing the electric field vector ({right arrow over (E)}) and the direction of maximum radiation. The H-plane is similar, though orthogonal to the E-plane. An exemplary radiation pattern for a particular type of antenna will be described in more detail below.
The directivity, gain and polarization of an antenna may be computed with knowledge of an antenna's radiation pattern. The “directivity” of an antenna may be generally defined as the direction of maximum radiation. For example, the radiation pattern of most directional antennas may include one main lobe (pointing in the direction of maximum radiation), but may also include several smaller side lobes (due, e.g., to reflections or cross-polarizations within the antenna). These side lobes generally detract from the overall performance of the antenna by reducing the amount of EM energy radiated in the intended direction. The “gain” of a directional antenna may be defined as the directivity multiplied by the radiation efficiency of the antenna. Thus, the antenna gain will be less than the directivity for antenna designs, which provide less than 100 percent radiation efficiency (i.e., real antennas).
As noted above, electromagnetic fields are radiated from antennas as vector quantities. The behavior of the vector nature of an electromagnetic field is often referred to as the “polarization” or “polarization state” of an antenna. Most antenna designs used for Electromagnetic Compatibility (EMC) testing are linearly polarized, meaning that the electric (or magnetic) field components are confined to one plane. On the other hand, some antenna designs may exhibit an elliptical polarization, or radiation that is polarized predominantly in one plane with a slight cross-polarization component, which is out of phase with the principle component. In elliptical polarizations, the tip of the electric field vector may trace an elliptical pattern in any fixed plane intersecting, and normal to, the direction of propagation. An elliptically polarized wave may be resolved into two linearly polarized waves in phase quadrature, such that their polarization planes are at right angles to each other.
A dual-ridged horn antenna, or dual-ridged waveguide, is one example of a linearly polarized antenna. When heavily loaded, a dual-ridged waveguide can provide a significantly broad bandwidth (e.g., from about 1 GHz to about 18 GHz). As shown in FIGS. 1 and 2, a dual-ridge horn 100 may include a pair of antenna elements 110 (often referred to as “ridges” or “fins”) arranged opposite one another within a rectangular-shaped horn antenna. Each of the antenna elements 110 may have a substantially convex inner surface 112 and a substantially straight outer surface 114. In most cases, each of outer surfaces 114 may be fixedly attached to one of the sidewalls 120 forming horn antenna 100. When coupled together, sidewalls 120 may form a rectangular-shaped cone structure having a substantially larger aperture 130 than base 140. In some cases, a rectangular-shaped box (or “cavity structure”) 150 may be coupled to the similarly shaped base 140. The cavity structure may include a power connector 160 for supplying electrical current from a power source (not shown) to the pair of antenna elements via a coaxial transmission line (not shown). A conductive feed line 170 may also be provided to transfer the electrical current from the coaxial transmission line to the pair of antenna elements 110 of the horn antenna. The transition from the transmission line to the conductive feed line 170 is an important part of the horn in that it comprises part of the horn's feed region (i.e., the region at which power is supplied to the antenna elements). When power is supplied, the inner surfaces 112 of the antenna elements function as tapered waveguides to guide the radiated energy as it travels from base 140, through the “throat” of the horn antenna, and out through the “mouth” or aperture 130 of the antenna.
Conventional broadband horn antennas used in EMC test systems typically demonstrate an operating frequency range of approximately 1 GHz to 18 GHz. However, the upper frequency range is often beset with anomalies in the radiation pattern. As the frequency increases, these so-called anomalies may surface as an increase in side lobes (180, FIG. 3), an increase in back lobes (185), or even the splitting or modification of the main lobe (190). At the highest end of the frequency range, the radiation pattern is primarily controlled by the characteristics of the feed region of the horn. For example, as the frequency increases, electromagnetic energy tends to pull further and further away from the inner surfaces 112 of the antenna elements. Such pulling away begins at the “mouth” of the antenna and gradually increases until the energy begins to pull away at successfully shorter distances from the feed region. This tends to increase the amount of transverse current on the inner surfaces of the antenna elements, and higher-order modes to develop in the feed region. In some cases, the higher-order modes may detract from the intended radiation pattern by redirecting a substantial amount of energy into side lobes and/or back lobes.
At least one horn antenna design has been proposed in which a device for suppressing higher-order modes in the feed region has been incorporated. This device essentially amounts to a strip-like conductor placed in between the two ridges (i.e., antenna elements 110) of the horn antenna. This device is somewhat effective in suppressing higher-order modes and represents an improvement over earlier designs, which failed to address mode suppression altogether. However, due to tolerance constraints, the strip-like conductor fails to provide a viable solution to the radiation pattern anomalies at all times. For example, the strip-like conductor must be symmetrically arranged between the ridges with an exceptionally tight tolerance. Not only is this difficult to accomplish in dual-ridged antenna designs, but it becomes exceedingly so in quad-ridged antenna designs, due the even tighter space constraints imposed within the feed region.
A quad-ridged horn antenna is basically a dual-polarized version of a dual-ridged horn antenna and functions, in the ideal case, by exploiting the orthogonality of two modes in the quad-ridged waveguide. In other words, quad-ridged horn antennas combine two linearly polarized waves to produce an elliptically polarized waveguide. As noted above, an elliptically polarized wave is polarized predominantly in one plane with a slight cross-polarization component that is not in phase with the principle component. Though careful design may minimize the cross-polarization component, it cannot completely eliminate it. In a practical situation, coupling between the two modes, especially in the feed region, is inescapable and detracts from the horn antenna's performance. Because of various difficulties in implementing the feed region (e.g., space constraints), quad-ridged horns have not been able to provide the same bandwidth as dual-ridged, single-polarization horns. At best, conventional quad-ridged horn antennas may provide an operating frequency range of about 1 GHz to about 10 GHz.
In addition to reduced operating frequency range, conventional dual- and quad-ridged horn antennas are often plagued with anomalies in the lower frequency ranges. At the low end of the operating frequency range, the characteristics of the “mouth” tend to control the radiation pattern of the dual- and quad-ridged horn antennas. Moreover, reflections from the “mouth” may cause great fluctuations in the “throat” impedance and significant pulse distortion. At the lowest end of the frequency range, current may flow around the edge of the “mouth” to increase the number of side lobes and back lobes in the radiation pattern. This may ultimately destroy the unidirectional properties of the horn.
Therefore, a need exists for improved dual-ridged and quad-ridged horn antenna designs that provide enhanced control of the intended radiation pattern over a maximized operating frequency range.